Distributed component model

ABSTRACT

A distributed component model for creating a scalable and available distributed application is disclosed. The distributed component model provides for an application schema to be declaratively defined to include a module having a component. The schema includes a corresponding definition construct in a technology agnostic manner. The corresponding definition construct is declaratively defined to include metadata to control scaling and availability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of Ser. No. 13/071,982 filed Mar. 25, 2011, entitled “DISTRIBUTED COMPONENT MODEL,” which is incorporated herein by reference.

BACKGROUND

Distributed computing applications are often deployed into environments having a multitude of different technologies and services that are used to form building blocks of the applications. Examples of distributed applications are legion and can include enterprise applications such as line of business or LOB, billing systems, customer relationship management or CRM, enterprise resource planning or ERP, business intelligence, human resource management, manufacturing, inventory control applications, and others. Such applications include components that are typically distributed across tiers in a computer network. Also, some applications are intended to run in a cloud computing environment, others are intended to run on the premises of the entity or user, and others are intended to span these environments. Further, the environment may change as an application evolves, the number of users change, or the locations of the users become dispersed.

One desirable characteristic of a distributed application is its ability to scale, or to cost-effectively change with the enterprise. Existing program models do not aim to support the development of scalable distributed applications. Typical component models are designed for desktop applications and are tier and technology specific. A distributed application is typically comprised of a set of distinct components, spread across tiers, which interact to perform work. While the components are virtualized, the relationship between the components is not. A physical wiring of components during runtime interaction is typically statically determined or otherwise hard-coded in this framework, which can place limits on the ways in which the application can be scaled or even on the application's overall ability to scale. While working with such models, many developers try to avoid writing stateful components because they are difficult to scale, but in making this choice the developer sacrifices benefits of other approaches, such as the natural expression of application logic.

Current techniques of state portioning and replication are limited to high-end developers and are implemented by technologies of databases and distributed caches. There is no program model, however, that makes these techniques and technologies approachable and mainstream for developers to use in writing and scaling application state logic.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

The present disclosure is directed to a distributed component model for writing and executing both stateless and stateful components that are both scalable and highly available. The model also includes a set of cross cutting capabilities for components such as tenant-aware billing, metering, throttling, and the like. Stateless components are made scalable and available with techniques such as cloning or process replication, and stateful components are made available with techniques such as partitioning and state replication. Scalability is affected through the mainstream programming modifications to component metadata and through multiple programs models, which avoids difficult and costly re-architecture of stateful applications. Additionally, the distributed component model is extensible and technology neutral, and it can be used to expose component capabilities over standard Web protocols.

In one example, the disclosure relates to a distributed component model for creating a distributed application. The distributed component model provides for an application schema to be declaratively defined to include a module having a component. The schema includes a corresponding definition construct in a technology agnostic manner. The corresponding definition construct is declaratively defined to include metadata to control scaling and high availability.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

FIG. 1 is a block diagram illustrating an example computing device for running, hosting, or developing a distributed application.

FIG. 2 is a block diagram illustrating a schema of an example distributed application according to a distributed component model.

FIG. 3 is a block diagram illustrating a state machine for a module of the example distributed application of FIG. 2.

FIG. 4 is a block diagram illustrating a state machine for a component of the example distributed application of FIG. 2.

FIG. 5 is a block diagram illustrating an application definition of the distributed application of FIG. 2.

FIG. 6 is a block diagram illustrating an example application lifecycle of the distributed application of FIG. 2.

FIG. 7 is a block diagram illustrating scale out and high availability of a stateless module.

FIG. 8 is a block diagram illustrating scale out and high availability of a stateful module.

FIG. 9 is a block diagram illustrating a method of creating the distributed application of FIG. 2.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. It is to be understood that features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 illustrates an exemplary computer system that can be employed in an operating environment such as a distributed computing system or other form of computer network and used to host or run a distributed application included on one or more computer readable storage mediums storing computer executable instructions for controlling a computing device or distributed computing system to perform a method. The computer system can also be used to develop the distributed application and/or provide a serialized description or visualized rendering of the application.

The exemplary computer system includes a computing device, such as computing device 100. In a basic configuration, computing device 100 typically includes a processor system having one or more processing units, i.e., processors 102, and memory 104. Depending on the configuration and type of computing device, memory 104 may be volatile (such as random access memory (RAM)), non-volatile (such as read only memory (ROM), flash memory, etc.), or some combination of the two. This basic configuration is illustrated in FIG. 1 by dashed line 106. The computing device can take one or more of several forms. Such forms include a person computer, a server, a handheld device, a consumer electronic device (such as a video game console), or other.

Computing device 100 can also have additional features or functionality. For example, computing device 100 may also include additional storage (removable and/or non-removable) including, but not limited to, magnetic or optical disks or solid state memory, or flash storage devices such as removable storage 108 and non-removable storage 110. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any suitable method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Memory 104, removable storage 108 and non-removable storage 110 are all examples of computer storage media. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile discs (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, universal serial bus (USB) flash drive, flash memory card, or other flash storage devices, or any other medium that can be used to store the desired information and that can be accessed by computing device 100. Any such computer storage media may be part of computing device 100.

Computing device 100 includes one or more communication connections 114 that allow computing device 100 to communicate with other computers/applications 115. An example communication connection can be an Ethernet interface. In some examples, the computing device can also have one or more additional processors or specialized processors (not shown) to perform processing functions offloaded from the processor 102. Computing device 100 may also include input device(s) 112, such as keyboard, pointing device (e.g., mouse), pen, voice input device, touch input device, etc. Computing device 100 may also include output device(s) 111, such as a display, speakers, printer, or the like.

The computing device 100 can be configured to run an operating system software program and one or more software applications, which make up a system platform. In one example, the computing device 100 includes a software component referred to as a managed, or runtime, environment. The managed environment can be included as part of the operating system or can be included later as a software download. Typically, the managed environment includes pre-coded solutions to common programming problems to aid software developers to create applications, such as software programs, to run in the managed environment. An example of a managed environment can include an application framework sold under the trade designation .NET Framework available from Microsoft, Inc. of Redmond, Wash. U.S.A.

FIG. 2 illustrates a schema 200 for a distributed application 202. The schema 200 generally describes the distributed application 202 constructed according to a distributed component model in an application framework. The distributed component model defines the schema 200 in abstractions including application, module, and component. The distributed application 202 includes one or more modules 204 a-204 n, and each module 204 a-204 n includes one or more components 206 a-206 n. Each component 206 a-206 n can specify imports and exports and includes metadata and artifacts. Application 202 is hosted in an application fabric that, in one example, provides the capabilities to deploy, run, and manage distributed applications that are long running, stateful, and allow high availability and elastic scale. At runtime (also referred to as execution time), the application framework provides the connections between the components 206 a-206 n of the application 202, described logically in the distributed component model.

The distributed application 202 has an identity and is a unit of deployment and management in the application fabric. When deployed, the application 202 spans tiers in the environment. Examples of tiers can include a client tier in many forms; a web tier, which is typically stateless, that can be available all of the time; a worker tier, including stateless and stateful components, that provides much of the logic of the application 202; and a storage tier that can be located on premises, in the cloud environment, or in a combination of the two. (Stateless can include abstractions that have no state or externalize state.) In one example, the application 202 is deployed to application fabric host farms. Physically, the application includes a package containing an application manifest that describes the compositional structure of the application, implementation details, configuration, and application artifacts. The application 202 in the example also corresponds to a tenant in the distributed component model and can include tenants of its own. This provides a layered and extensible tenant model that can be used for billing, throttling, metering, and the like.

Modules 204 a-204 n are a logical grouping of one or more components 206 a-206 n. For example, modules 204 a-204 n are each a tier-specific unit of hosting, which includes aspects of activation, scalability, availability, and isolation. Components 206 a-206 n in module 204 a are deployed to the same host or single process and are typically co-located in the execution environment. The components 206 a-206 n often leverage the co-location for data affinity or the like.

The distributed component model provides a formal module execution and deployment lifecycle. FIG. 3 illustrates a state machine 300 for the module lifecycle. State changes are triggered in the environment or some other event in the system. The module performs an appropriate action when a state change occurs. The following Table I describes the module states with reference to the elements of the state machine 300, and describes an event or trigger that takes a start state to a target state.

TABLE I Source State Trigger/Event Target State Uninitialized 302 All component Initialized 304 definitions loaded Initialized 304 Request to load Loading 306 component runtime objects for composition Initialized 304 Deploy is called on Installing 308 module Initialized 304 Un-deploy is called on Uninstalling 310 module Loading 306 Runtime components Loaded 312 created and components ready for composition Loading 306 Request to unload Unloading 314 module Loading 306 Exception during load Faulted 316 Loaded 312 Start signal from host to Starting 318 module to start components Loaded 312 Host calls Unload on Unloading 314 module Starting 318 All components Started 320 successfully started Starting 318 Exception during startup Faulted 316 sequence Started 320 Hosting environment Stopping 322 calls stop on module Started 320 Exception thrown after Faulted 316 module has started Stopping 322 All components stopped Stopped 324 successfully Stopping 322 Exception thrown during Faulted 316 stopping the module Stopped 324 Hosting environment Starting 318 triggers start Stopped 323 Hosting environment Unloading 314 triggers unload of module Faulted 316 Faulted component faults Faulted 316 again Faulted 316 Hosting environment Stopping 322 triggers stop when module is in faulted state Faulted 316 Hosting environment Unloading 314 called unload while the module is in faulted state Unloading 314 All components in the Unloaded 326 module were unloaded successfully. Installing 308 Module successfully Installed 330 installed Installed 330 Host triggers uninstall of Uninstalling 310 the module Uninstalling 310 Module successfully Uninstalled 332 uninstalled

In one example, each module is a tier-specific unit of hosting. Each module 206 a-206 n can have an associate role such as a worker in a worker module or web in a web module. Several types of modules can exist in the distributed component model, and the module types correspond to the capabilities of the hosting environment. Such module types can include stateless, Web, stateful, browser, and storage. A stateless module can include stateless components on modules capable of hosting stateless components, such as worker role or an application fabric role. The Web module is hosted on web hosts. A stateful module includes stateless components and can be hosted in a fabric-aware host. The browser module can be hosted in a Web browser. The storage module can be hosted on storage servers such as, for example, SQL (structured query language) database servers.

The modules 204 a-204 n can also include cross-cutting aspects, which include aspects and filters to lift cross cutting concerns such as logging, throttling, and metering, and the like out of the application logic. In the distributed component model, the module 204 a can have zero or more cross cutting aspects associated with it. In one example, the cross cutting aspects reflect the core Aspect Oriented Programming (AOP) idioms. For example, each aspect can include zero to n advices, policy metadata, and a jointpoint for which it is being invoked. Also, each advice can include zero to n pointcuts and zero to n subscribers. The pointcut is a predicate, i.e., a LINQ expression, evaluated at a jointpoint. (Language Integrated Query (LINQ) is a trade designation for a data query component available from Microsoft, Inc., in the Microsoft .NET Framework that adds native data querying capabilities to .NET languages, such as C-sharp (C#)). Upon pointcut evaluation, all Before, After, and Around advices are invoked.

A component, such as component 206 a, in the distributed component model is a unit of technology encapsulation, extensibility, composition, and reuse. (Components 206 a-206 n are distinguishable from a common language runtime object/type or with components in other technologies like component object model or distributed component object model, i.e., “COM/DCOM.”) The component 206 a encapsulates a certain technology. Such technologies can include, for example, web application technologies or application programming interfaces for building connected, service-oriented applications. More than one component type can be developed for a given technology. For example, the application 202 could include a web application component and a service component in the web tier, a code component, a cache component, and a workflow component in the worker tier, and various storage components (such as tables or queues) and an SQL database component in the storage tier. In one example, the component 206 a is a wrapper around a set of functionality. This wrapper hides the implementation details of the component yet exposes the functionality and dependencies that can allow loose coupling between service provider and consumers.

The distributed component model provides a formal component execution and deployment lifecycle. FIG. 4 illustrates a state machine 400 for the component lifecycle. State changes are triggered in the environment or some other event in the system. The component performs an appropriate action when a state change occurs. The following Table II describes the component states with reference to the elements of the state machine 400, and describes an event or trigger that takes a start state to a target state.

TABLE II Start State Trigger/Event Target State Uninitialized 402 This is the initial state of Initialized 404 the Component. Initialized 404 Request to load Loading 406 component runtime Initialized 404 Host triggers deployment Installing 408 of the component Initialized 404 Host triggers un- Uninstalling 410 deployment of the component Loading 406 Runtime component Loaded 412 created and component ready for composition Loading 406 Request to unload Unloading 414 component Loading 406 Exception during load Faulted 416 Loaded 412 Component start Starting 418 triggered Loaded 412 Host triggers unload Unloading 414 Starting 418 Component successfully Started 420 started Starting 418 Exception during startup Faulted 416 sequence Started 420 Hosting environment Stopping 422 calls stop on component Started 420 Exception thrown after Faulted 416 component has started Stopping 422 Component stopped Stopped 424 successfully Stopping 422 Exception thrown during Faulted 416 stopping the component Stopped 424 Hosting environment Starting 418 triggers start Stopped 424 Hosting environment Unloading 414 triggers unload of component Faulted 416 Faulted component faults Faulted 416 again Faulted 416 Component started after Started 420 faulting by the hosting environment Faulted 416 Hosting environment Stopping 422 triggers stop when component is in faulted state Faulted 416 Hosting environment Unloading 414 called unload while the component is in faulted state Unloading 414 Component was Unloaded 426 successfully unloaded. Installing 408 Component was Installed 430 successfully deployed Installed 430 Hosting environment Uninstalling 410 triggers uninstall of the component Uninstalling 410 Component was Uninstalled 432 successfully uninstalled

The components 206 a-206 n can include artifacts and define the metadata at runtime. In one example, a component metadata can include a security configuration. A component artifact can include configuration files, binaries, user code, and the like. Component metadata and artifacts can be captured in the application manifest and are made available to the component at runtime.

Components 206 a-206 n can export, i.e., offer, a set of capabilities and can import, i.e., use, a set of capabilities. A component can export a capability or a service that can be consumed by other components. Also, a component can import a capability or a service for consumption from another component 206 n in the application 202 or from an external service. Thus, component exports imports and component imports are the mechanisms by which the components 206 a-206 n are stitched together to form the application 202. Stitching may be described at the design stage or can be dynamic in that available exports can be discovered, imported, and used at runtime. In either case, the stitching is a logical expression of a component relationship. The procurement of proxies and the resolution of physical addresses to get two component instances communicating are brokered at runtime.

The component export is a declarative specification of a capability offered at runtime. The component export can also represent an external piece of the application that is not part of the application being modeled. For example, an export can represent a message queuing technology such as one offered under the trade designation of MSMQ available from Microsoft, Inc. or a web service such as one offered under the trade designation of Amazon Web Services (AWS) available from Amazon.com of Seattle, Wash. U.S.A. The component export also includes runtime logic to manufacture proxies that component imports can use. Component exports can be made visible at different scopes such as within the application or externally. Similar to components 206 a-206 n, component exports are associated with metadata and artifacts. Within the application 202, an export can be identified by a contract and a logical address. The shape and semantics of the contract can be related to the technology used and is opaque to the distributed component model. In one example, component exports are reusable, and independent software vendors can provide the components exports as a library. The component export includes metadata regarding cardinality, which specifies the number of imports acceptable to the component: none, one, or more than one.

A component import is also a declarative specification of a capability consumed by an instance of the component 204 a. Component imports are satisfied by component exports that match the criteria of the component import, and the matching criteria is expressed as a declarative predicate on the component import. The predicate is evaluated to match/select from a set of available component exports visible scope of the component requesting the import. In one example, the component 206 a will determine a match based on the name of the predicate, but the component can also determine a match on import/export metadata specified by an author of the component 206 a or the application 202. The component import typically includes metadata regarding to cardinality, which specifies the number of exports acceptable to the component 206 a: none, one, or more than one.

The distributed component model provides a mechanism for declaratively describing and constructing the distributed application 202 in an application definition. The application definition describes a form of a type system that captures the components 206 a-206 n within the application 202, the producer-consumer relationships between the components 206 a-206 n, and any external components or services consumed by components 206 a-206 n in the application 202. The application definition describes the configuration and constraints of the components as well as component dependencies, interrelationships, and interactions of the distributed application in a declarative manner. The application definition also provides the ability to schematize and extend the compositional structure and metadata in a format/representation agonistic manner. It can be use to validate the compositional structure of the distributed application 202 as well as enforce the composition structure at runtime. Such a representation of compositional structure of an application having complex interactions among a set of distributed components provides the ability to reason over an application lifecycle and can be used to scale the distributed application 202 in a distributed environment.

FIG. 5 illustrates an example application definition 500. The application definition 500 includes the constructs of an application definition 502, one or more module definitions 504 a-504 n, and one or more component definitions 506 a-506 n for each module definition 504 a. The arrangement of the definition constructs resembles an ontology similar to the distributed application schema 200 as a definition tree. The root of the definition tree is the application definition 502. Each of the module definitions 504 a-504 n corresponds with a particular module of the module 204 a-204 n. Each component definition 506 a-506 n corresponds to one of the components of components 206 a-206 n. Additional constructs are included in the definition tree depending on the particular features of the components. For example, each component using an import includes a component import definition 508, and each component offering an export includes a component export definition 510. Each component that provides an aspect, such as a cross-cutting concern, includes a component aspect definition 512.

The definition constructs include a declarative description of the corresponding application, module, and component. Each definition construct includes associated metadata that further describes the construct. In one example, the component definitions 506 a-506 n for the application each include a common set of metadata that describe aspects of the corresponding component. Similarly, the module definitions 504 a-504 n for the application each include a common set of metadata that describe aspects of the corresponding module. The component import definitions 508, the component export definitions 510, and the component aspect definitions 512 can each include common sets of metadata. In addition to the common set of metadata, each component definition can specify component-specific metadata, which is also true for module, component export, component import, and component aspect definitions. In one example, the component-specific metadata is opaque to the distributed component model and is understood by the component 206 a and other components that consume it. The application definition 500 in the distributed component model is validated to enforce component interrelationship and metadata. Each definition construct can also specify custom validation logic against the application definition.

FIG. 6 illustrates how the application definition 500 is created and is used through an application lifecycle 600. The distributed application 202 is constructed during the application design phase at 602. The distributed application 202 is constructed as per the schema 200 prescribed by the distributed component model. The output of the design phase 602 is a serialized application package that contains the application manifest and the artifacts that make up the different components 206 a-206 n. The application package is staged in an application fabric repository during an application staging phase at 604. The application package is posted to an end point on which an application farm fabric manager is listening. Once the distributed application 202 is posted, the application fabric farm manager shreds the application package. The application farm manager will access the artifacts for each component 206 a-206 n according to the application manifest and stores them in the application fabric repository. The application farm manager will also expose the application hierarchy as a Representative State Transfer (REST) resource that can be accessed by other applications or by the component code themselves. The distributed application 202 stored in the application fabric repository is deployed to a host farm during the deployment phase at 606. In order to deploy the distributed application 202 to the host farm, the farm manager will look at the application manifest and deploy the appropriate modules 204 a-204 n within the application to a corresponding set of nodes within the host farm. During an application initialization phase at 608, the various different modules 204 a-204 n deployed to the nodes are loaded into the host process and the components 206 a-206 n within the modules 204 a-204 n start executing. If the component is a service, the component will create the end point and start listening on the end point.

The distributed component model provides developers and enterprises the ability to cost-effectively build, run, and evolve the distributed application 202. Both stateful and stateless components can be developed using familiar technologies, emerging technologies, and custom paradigms for specific domains. The components 206 a-206 n can be stitched together either statically or dynamically to form the application 202. Cloning, replication, and partitioning are supported within the application 202, as is the ability to make architectural tradeoffs such as among consistency, availability, and tolerance of “partitions” (such as describe in Brewster's CAP Conjecture).

The distributed component model provides for scalable applications to include the techniques of cloning, replication, and partitioning. Different techniques may apply to different parts of the application 202, which may change over time as the application grows. For example, cloning is a relatively straightforward technique, but in certain technologies it is exclusively suited for stateless components. Replication is currently an effective technique for stateful components, but it can be complex and limited. For example, the amount of state can grow during the life of the application 202 such as in the form of user sessions or cached data that are replicated across machines, or a row-locking scheme in a shared store that becomes the bottleneck to the performance of the application 202. In order to address the issue of growing state, a developer may choose to partition one or more components, which previously involved a costly and difficult re-architecture of the application 202.

In order to avoid a costly re-architecture, the application 202 is initially designed in a distributed component model to support partitioning, which can be used regardless of whether application growth is anticipated. Design patterns and use of a distributed composition runtime can make intra-component wiring immune to otherwise invasive changes such as sharding, which is typically know as horizontal partitioning of a database, and component partitioning. Partitioning is made available in the application 202 and then is activated as desired. The application 202 can be readily designed to map the partitions to machines as well. Additionally, the developer can retain flexibility about whether a component 206 a or the entire application 202 runs on premise or in a cloud computing environment. As the costs of infrastructure change over time, the architecture of the application 202 can naturally evolve to take advantage of the relative cost changes.

Each module 204 a can be a logical grouping of related components 206 a-206 n for the purposes of co-location and partitioning. Components 206 a-206 b grouped together within a module can run within the same application domain. For example, two or more components 206 a-206 n can be co-located if they abide by the same partitioning scheme. In a partitioned module, each part is independent of the others and hence receives its own application domain within which the set of co-partitioned components for the corresponding part will run. The components 206 a-206 n within a module, such as module 204 a, can communicate via direct method invocations. Across modules 204 a-204 n, components communicate by sending messages. A module type can correspond to the capability of the host. For example, a stateless component, such as a web role, is hosted in a stateless module. Execution environments for modules include web and worker roles for stateless components and a fabric role for stateful components.

During runtime, the distributed application framework can monitor the application 202 to diagnose and repair issues as well as meter the use of the components 206 a-206 n. The distributed component model can elastically allocate and reclaim resources to support a fluctuating demand. Further, the distributed application framework provides for the ability to later partition the application 202, co-locate partitioned components 206 a-206 n, change a mapping of partitions to a physical infrastructure, and shard a database without costly re-architecture.

In one example, an application fabric available under the trade designation of AppFabric can run on premise, such as a server operating system available under the trade designation of Windows Server, and in a cloud environment having a cloud computing or cloud services operating system available under the trade designation Windows Azure, all available from Microsoft, Inc., allowing entire applications (or components within them) to be deployed to either environment or a combination the two. Web roles, workflow, and the like can be built using Windows Communication Foundation (WCF) and Windows Work Flow (WF) available from Microsoft, Inc.

In one example, a distributed application manifest provides the above-described distributed component model in definition constructs expressing the component configurations and their interrelationships to each other and interactions in a technology and format agnostic manner. The manifest is a serialized form of the application definition 500 and captures the entire structure of the application 202. In one example, the manifest is format agnostic and can be serialized in a variety of formats, which can include scripting languages such as extensible markup language (XML), extensible application markup language (XAML), JavaScript object notation (JSON), or binary JSON (BSON) and many others now know or yet to be created. The following example distributed application manifest is serialized in JSON:

{ “Name”: “MyApp”, “Id”: “622BN4TFQB3UHFEERJGFXPVX4A”, “BaseUri”: http://MyApp.cloudapp.net/, “SelfLink”: “...”, “Version”: “1.0.0.100” , “References”: [   {“Type”: “DistributedList”,...}, {“Type”:“TaskScheduler”,...}, {“Type”:“CloudQueue”,...},       {“Type”: “WCFService”,...} ], “ModuleDefinitions”:  [    {“Name”: “MyWebModule”, Type” : “Web”, “InstanceCountHint”: 2, “Components”: [ {...}] },    {“Name”: “MidTierModule”, “Type” : “Stateful”, “InstanceCountHint”: 2,    “IsolationLevel”: “Process”, “MachineSize”: “Large”,    “PartitionPolicy”: { “Type”: “RangePartitionPolicy”, “Keys”: [ “A-G”, “H-M”,“N-Z”] },         “ReplicaCountHint”: 2, “ReplicationFormat”: “JSON”, “WriteQuorum”: 1,         “Components”:     [        {“Name”: “MovieProcessor”, “ModuleAffinity”:         “Stateful”, ... “Imports”:         [           {“Name”: “DistributedList”, “Cardinality”: “ExactlyOne”, “InstancingPolicy”:              “Pooled”, “Constraint”: {...} } },           {“Name”: “NewMovies”,“Cardinality”: “AtleastOne”,“InstancingPolicy”:              “Singleton”,“Constraint”: {...} } },           {“Name”: “MovieService”,“Cardinality”: “AtleastOne”,“InstancingPolicy”:              “Singleton”,“Constraint”: {...} } },           {“Name”: “TaskScheduler”,“Cardinality”: “AtleastOne”,“InstancingPolicy”:              “Singleton”,“Constraint”: {...} } },         ],        }         ]    }  ...  ]  ... }

The manifest includes the application definition 502, the module definitions 504 a-504 n, component definitions 506 a-506 n, component exports 508, component imports 510, and component aspect definitions 512. In the example, the module definitions 504 a-504 n include metadata on instances, partitions, and replicas. A stateless module definition can include a declaratively defined instance count that control the number of module instances and describes the scalability and high availability (often referred to as “HA”) characteristics of a stateless module and its corresponding components. A stateful module definition can include a declaratively defined instance count, a partition policy, and a replica count to describe the scalability and high availability characteristics of a stateful module and its corresponding components. In order to evolve or scale the application, a developer adjusts the counts and policies within the metadata of the module definition to a selected amount.

FIG. 7 illustrates scale out and high availability of a stateless module 702, which can correspond with module 304 a for this example. The corresponding module definition 704 of the stateless module 702 includes an instance count 706 in the metadata. The instance count 706 controls the number of module instances, i.e., the scale out and high availability characteristics of the stateless module 702. The example module definition 704 includes an instance count 706 of 3, and thus 3 instances of the stateless module, i.e., instances 708 a, 708 b, 708 c, are created at runtime. For example, the module definition 704 can include metadata regarding “Enable High Availability,” which indicates if the module can be made highly available. Additionally, the module definition can include metadata regarding “Instance Count Hint,” which specifies the number of instances of the stateless modules such as 708 a to create at runtime.

FIG. 8 illustrates scale out and high availability of a stateful module 802, which can correspond with module 304 n for this example. The corresponding module definition 804 of the stateful module 802 includes an instance count 806, a partition policy 808, and a replica count 810 in the metadata. The instance count 806 controls the number of module instances and thus scale out. The partition policy 808 controls the number of partitions assigned to a given module instance. The replica count 810 controls the high availability and determines the number of replicas 816 to each partition. The example module definition 804 includes an instance count 806 of 3, i.e., instances 812 a, 812 b, 812 c. The example partition policy 808 assigns 4 partitions 814 to each instance 812 a, 812 b, 812 c, and the example replica count assigns 2 replicas 816 to each partition. The instances 812 a, 812 b, 812 c, partitions 814, and replicas 816 are created at runtime.

FIG. 9 illustrates an example method 900 of creating a distributed application in a distributed component model. An application schema including a stateless module having a stateless component and a stateful module including a stateful component is declaratively defined at 902. The application schema corresponds with a technology agnostic application definition having a stateless module construct corresponding with the stateless module and a stateful module construct corresponding with the stateful module. A selectable instance count for controlling a number of instances of replication of the stateless module through at least one of cloning and a replication process is declaratively defined with the stateless module at 904. Selectable metadata to control scaling and high availability with an instance count, a partition policy, and a replica count is declaratively defined within the stateful module construct at 906.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A method of creating a distributed application in a distributed component model, comprising: declaratively defining an application schema including a module having a component, the schema includes a corresponding definition construct expressing configurations in a serialized form of the distributed application and the module is a tier-specific unit of hosting; declaratively defining within the module of the corresponding definition construct by adjustably programmable counts and policies within metadata to control scaling and availability of the distributed application in the serialized form of the distributed application; and enforcing a compositional structure of the distributed application during runtime.
 2. The method of claim 1 wherein the schema includes a plurality of modules, and each module includes a plurality of components.
 3. The method of claim 2 wherein the definition construct includes a plurality of module definitions corresponding to the plurality of modules and each module definition includes a plurality of component definitions corresponding to the plurality of components.
 4. The method of claim 3 wherein the plurality of modules include a stateless module having a stateless component.
 5. The method of claim 4 wherein the stateless component is replicated through cloning.
 6. The method of claim 4 wherein the stateless modules correspond with module definitions having selectable metadata regarding instance count to specify the number of instances of the stateless module to create at runtime.
 7. The method of claim 4 wherein the plurality of modules include a stateful module having a stateful component.
 8. The method of claim 7 wherein the stateful module includes a corresponding stateful module definition having selectable metadata including an instance count, a partition policy, and a replica count.
 9. The method of claim 8 wherein the instance count controls a number of module instances, the partition policy controls a number of partitions assigned to the module instances, and the replica count controls a number of replicas for each partition.
 10. The method of claim 1 comprising: validating component interrelationships of the distributed application during runtime.
 11. The method of claim 1 wherein the definition construct is serialized from a scripting language.
 12. The method of claim 1 comprising: including the definition construct within an application manifest.
 13. The method of claim 1 wherein the application constructs further include a component export definition, a component import definition and a component aspect definition corresponding with the component.
 14. A computer readable storage medium storing computer executable instructions for controlling a computing device to: declaratively define an application schema including a module having a component, the schema includes a corresponding definition construct to express configurations in a serialized form of a distributed application in a distributed component model and the module is a tier-specific unit of hosting; declaratively define within the module of the corresponding definition construct by adjustably programmable counts and policies within metadata to control scaling and availability of the distributed application in the serialized form of the distributed application; and enforce a compositional structure of the distributed application during runtime.
 15. The computer readable storage medium of claim 14 comprising computer executable instructions to validate component interrelationships of the distributed application during runtime.
 16. The computer readable storage medium of claim 14 including computer executable instructions to apply the definition construct to validate a compositional structure of the distributed application.
 17. The computer readable storage medium of claim 14 wherein the application schema includes a plurality of modules, and each module includes a plurality of components.
 18. A system for creating a distributed application in a distributed component model, the system comprising: a processor; and memory comprising instructions executable by the processor to: declaratively define an application schema including a module having a component, the schema includes a corresponding definition construct to express configurations in a serialized form of the distributed application and the module is a tier-specific unit of hosting; declaratively define within the module of the corresponding definition construct by adjustably programmable counts and policies within metadata to control scaling and availability of the distributed application in the serialized form of the distributed application; and enforce a compositional structure of the distributed application during runtime.
 19. The system of claim 18 comprising instructions executable by the processor to apply the definition construct to validate a compositional structure of the distributed application.
 20. The system of claim 18 wherein the definition construct is serialized into an application manifest. 